Whitaker's eighth argument defending Sola Scriptura can be summarized (with a small amount of liberty) as follows:
P1. The oral revelation given to the patriarchs did not require the authority of the Church to authenticate it; the patriarchs believed it upon receiving it by virtue of hearing God speak.
P2. The written revelation of the canon is of the same kind and authority as the oral revelation given to the patriarchs.
C. The canon does not require the authority of the Church to authenticate it to us; it should be received in the same manner as the patriarchs received oral revelation.
Whitaker seems to draw additional support for the immediate reception of God's words in P1. from an appeal to Romans 2:15, where Paul says that God's law is written on our hearts. This belief in the law comes, therefore, not from the testimony of the Church. (I would add that this is especially the case since the subject of Romans 2:15 is gentiles who have never heard the law before.) But if the law, which is natural, can be discerned without the Church, how much more the Gospel, which "transcends all nature, and therefore needs some greater kind of confirmation" (the greater confirmation, according to Whitaker, being the Holy Spirit).
I'd add my own support for P2. by noting that God communicates his Words through ordinary means. The patriarchs either audibly heard God speak to them or used the mind's "ear." In either case, physical processes (for even the latter arguably required some level of brain function) were utilized to obtain mental understanding. There doesn't seem to be a significant or functional difference between this and reading or hearing the Words of God in the written canon. If this process of knowing God's Word was valid for the patriarchs, why would it be invalid for Protestants?